Heating of foods in a microwave oven differs significantly from heating foods in a conventional oven. In a conventional oven, heat energy is applied to the exterior surface of the food and moves inward until the food is cooked. Thus, food cooked conventionally is typically hot on the outer surfaces and warm in the center.
Microwave cooking, on the other hand, involves absorption, by the food, of microwaves which characteristically penetrate far deeper into the food than does infrared (heat). Also, in microwave cooking, the air temperature in the microwave oven may be relatively low. Therefore, it is not uncommon for food cooked in a microwave oven to be cool on the surfaces and much hotter in the center. This makes it difficult to brown food and make it crisp. Therefore, it is difficult to make some food cooked in a microwave oven aesthetically pleasing.
In order to facilitate browning and crisping of food in a microwave oven, devices known as susceptors have been developed. Susceptors are devices which, when exposed to microwave energy, become very hot. By placing a susceptor next to a food product in a microwave oven, the surface of the food product exposed to the susceptor is surface heated by the susceptor and thereby becomes crisp and brown.
Many conventional susceptor structures have included a thin metal film, typically 60-100.ANG. of Aluminum, deposited on a substrate such as polyester. The metalized layer of polyester is typically bonded, for support, to a support member such as a sheet of paper board or corrugated paper.
Conventional susceptors have certain drawbacks. They undergo a process called breakup in which the electrical continuity of the thin metal film is lost during cooking. This is described in more detail in the Wendt et al U.S. Pat. No. 4,927,991. The result of the loss of electrical continuity is an irreversible loss in the susceptor's microwave responsiveness and a lower level of percent power absorption in the susceptor during cooking. Lower power absorption leads to lower susceptor cooking temperatures and a corresponding decrease in the susceptor's ability to crisp food.
In order to further discuss the relevance of this deterioration, some other relationships should be set forth. The complex dielectric constant .epsilon. of a material is defined as follows: EQU .epsilon.=.epsilon..sub.0 .epsilon..sub.r =.epsilon..sub.0 (.epsilon..sub.r -j.epsilon..sub.r ") Eq. 1
where
.epsilon..sub.0 is the permitivity of free space, 8.854.times.10.sup.-14 Farads/cm; PA1 .epsilon..sub.r is the complex relative dielectric constant of the susceptor, relative to free space; PA1 .epsilon..sub.r ' is the real part of the complex relative dielectric constant .epsilon..sub.r ; and PA1 .epsilon..sub.r " is the imaginary part of the complex relative dielectric constant .epsilon..sub.r. .epsilon..sub.r " is also known as the loss factor for the material.
As an example of conventional susceptor operation, a frozen food product could be placed on a susceptor. The susceptor and the food product could then be subjected to microwave energy. Since .epsilon..sub.r " (the imaginary part of the complex relative dielectric constant) of ice is very low, the frozen food product is initially a poor absorber of microwave energy. Therefore, the susceptor absorbs an excessive amount of the microwave energy and begins to deteriorate. Meanwhile, the frozen food product absorbs very little energy. This is undesirable. As the frozen food product thaws and starts absorbing microwave energy, the ability of the susceptor to absorb energy, and thereby surface heat the frozen food product, has already been deteriorated. Since this deterioration (i.e., the change in the electrical continuity of the susceptor) is generally irreversible, the susceptor is incapable of properly browning and crisping the food product.
In addition, as the susceptor deteriorates, it heats in a non-uniform fashion resulting in hot spots distributed along the surface of the susceptor. This results in uneven surface heating of the food products.
Further, as the susceptor deteriorates and the microwave transmissiveness of the susceptor increases, the food product may be subjected to an undesirable amount of dielectric heating. This can cause the food product to become tough or to attain other similarly unappealing qualities.
Therefore, there is a continuing need for the development of susceptor structures which are not plagued by the problems of thin metallic film-type susceptor structures.